add disqus_url to comment js block

By default, the `disqus_url` is not set in the `disqus_thread ` block, which is suggested to be set in the [disqus official docs][disqus-doc].

With `disqus_url` not set, if I opened my article in the browser in a dev environment before publishing, the page is rendered with url such as `http://localhost:8000/your-slug/`, and disqus will use that url as the `disqus_url`(explained in the [doc][disqus-doc]). 

Once set, the thread will use that `disqus_url` to generate links to the comments, that is, every comment, no matter put on product environment or dev env, will be set with the `localhost:8000` started url. What's more, the `disqus_url` [**cannot** be updated][disqus-url-update-policy] once set, you'll have the change the identifier(slug) to get rid of that `localhost:8000` url.

So, I change the `article` template of `not my idea` theme, and I recommend other theme maker to add the `disqus_url` settings, too.

  [disqus-doc]: http://help.disqus.com/customer/portal/articles/472098-javascript-configuration-variables "disqus official doc"
  [disqus-url-update-policy]: http://help.disqus.com/customer/portal/articles/735170-how-can-i-update-discussion-urls "how to update disqus url"